I guess if you go overboard you'd die but it seems with training it becomes easier to handle, Gokus body was in major pain from like X3 in the Saiyan Arc whereas in against Frieza he could handle far higher levels, 8 Gates is a definite kill.
That was because of Goku's power tho If Goku could use 8 Gates, It be far beyond the level of Naruto characters.
I mean if Akamaru used 8 gates it wouldn't do anything now would it? 8 Gates has to have some kind of power-scale to the one using it (I realize the manga says otherwise but I don't know, would Lee using 8 Gates at the same time make him as strong as Guy for instance?)
Also Kaio-Ken has no limits to increase. It could increase a person's power by 200X if one wanted. And it wouldn't obliterate them as long as they could handle it. You mentioned yourself how after Goku trained more he could easily handle 20X. Super Saiyan was obviously stronger, but had he wanted he could have increased Kaio-Ken to hundreds of thousands of times more power by Buu Saga.
8 Gates has a power limit, Kaio-Ken does not. It would be logical then that Kaio-Ken is more powerful.